A quadrupole electrode used in a mass spectrometer of the like comprises four electrodes 11, 12, 13 and 14 formed in such a manner that opposed surfaces are hyperbolic in their cross section as shown in FIG. 4, or four electrodes 11', 12', 13' and 14' formed so as to have a circular cross section as shown in FIG. 5 are disposed in a positional relationship adjusted so that the electrodes are located at predetermined intervals. When ions are fed into the center of the quadrupole electrode in the direction indicated by an arrow, it becomes possible to take out ions having a particular mass to charge ratio with a high accuracy from the opposite side of the quadrupole electrode. In such a conventional quadrupole electrode, the distance between the electrode rods should be kept so accurately that a very highly accurate work is required in assembling the quadrupole electrode and a long time are necessary for the assembly and adjustment of the quadupole electrode. Further, a change in the distance between the electrodes caused during the analysis should be minimized.
For example, Japanese Patent Laid-Open No. 30056/1983 describes the use of an electrode produced by subjecting a metallic material to extrusion or drawing into a V-shaped electrode for the purpose of reducing the weight of the electrode and, at the same time, improving the dimensional accuracy. Further, Japanese Patent Laid-Open No. 87743/1984 and Japanese Utility Model Laid-Open No. 64562/1985 describe the shape of electrode rods which are easy to assemble into a quadrupole electrode. Further, other various designs have been proposed in the art.
In the conventional quadrupole electrode, in order to bring the accuracy of the distance between the constituent electrodes to a predetermined value, it is a common practice to use a method which comprises manually assembling a quadrupole electrode, introducing a monitor gas for confirming the accuracy and repeating a check on the accuracy to correct the distance between the electrodes. According to the present invention, the constituent electrodes can be disposed with a high dimensional accuracy without any such troublesome work and the predetermined accuracy of the distance between the electrodes can be kept high during the use thereof.
The present invention provides a quadrupole electrode comprising two pairs of opposed electrodes, characterized in that the electrode rods are constituted of electrode rods which are made of an insulating ceramic and coated with a conductive metal, and are previously fixed with a predetermined dimensional accuracy.
The section of the opposed face of each electrode is a hyperbolic or circular. The ceramic constituting the electrode rod has a coefficient of thermal expansion of 9(.times.10.sup.-6 /.degree.C.) or less, more preferably a coefficient of thermal expansion of 4(.times.10.sup.-6 /.degree.C.) or less.
The present invention provides a process for producing a quadrupole electrode which comprises incorporating the above-mentioned four electrodes at predetermined intervals in such a manner that two pairs of the electrodes are arranged opposite to each other. In the production, the four electrodes are jointed to each other directly or through a jig.
Thus, the present invention has been made with a view to facilitating the formation of a quadrupole electrode with a high accuracy and a good reproducibility. In the present invention, a high accuracy within .+-.5 .mu.m can be attained in the distance between the electrodes and a change in the distance between the electrodes during the use thereof in the analysis can be minimized by using an insulating ceramic having a low coefficient of thermal expansion and subjected to high-accuracy working as the material of the electrode and, after coating the surface of the electrode with a conductive metal, assembling four electrodes, and incorporating the resultant quadrupole electrode in a mass spectrometer.
In order to improve the accuracy of assembling a quadrupole electrode and, at the same time, to shorten the time necessary for the adjustment of the accuracy, it is necessary to assemble at once the electrodes into a quadrupole electrode through reference planes finished with a predetermined accuracy. When a metal is used as the material of the electrode, however, there occurs a problem that the insulation between the electrodes cannot be maintained. This problem can be solved through the use of an insulating ceramic. Since ceramic has a low coefficient of thermal expansion and a light weight, it is advantageous in that the dimensional stability against a change in the temperature can be maintained and improved and the handleability is good. A ceramic having a coefficient of thermal expansion of 9(.times.10.sup.-6 /.degree.C.) or less suffices for this purpose, and use may be made of Si.sub.3 N.sub.4, sialon, mullire, SiC, AlN, Al.sub.2 O.sub.3, cordierire, quartz, etc.